The present invention relates generally to improved protective or hard coating compositions and coated articles. More specifically, the present invention relates to novel hard coatings produced from novel compositions containing two different inorganic components plus a third component, which contains cross-linkable organic functionality. Particularly, such coating compositions and coated articles employ silicone resin coating compositions which when applied to a substrate form a protective abrasion-resistant coating thereon.
For purposes of the present specification, a hard coating will refer to a coating that exhibits good mechanical properties, such as scratch-resistance and abrasion-resistance. There are many different approaches known in the art for producing hard coatings. These hard coatings are utilized in many different applications. For example, hard coatings are often used to protect furniture, as well as to protect various parts of an automobile.
The substitution of glass glazing with transparent materials which do not shatter or are more resistant to shattering than glass has increased. Such transparent glazing made from synthetic organic polymers is not utilized in public transportation vehicles, such as trains, buses, taxis and airplanes. Also, lenses for eyeglasses and other optical instruments, as well as glazing for large building employ shatter-resistant transparent plastics. The lighter weight of such transparent plastics is a further advantage. However, such transparent plastics have the drawback of being easily marred and scratched because of everyday contact with abrasives, such as dust, cleaning equipment and ordinary weathering. Continuous scratching and marring results in impaired transparency and poor aesthetics, requiring replacement. This includes polycarbonates such as Calibre(copyright) brand polycarbonate sold by The Dow Chemical Company and Lexano brand polycarbonate sold by General Electric Company. Attempts to improve the abrasion resistance of transparent plastics and to provide articles with hard coatings which are scratch resistant have employed coatings formed from mixtures of silica, such as colloidal silica or silica gel, and hydrolyzable silanes in a hydrolysis medium, such as alcohol and water, as taught in U.S. Pat. Nos. 3,708,225; 3,986,997 and 3,976,497.
It is also known in the art to produce hard coatings using sol-gel processes and techniques. It is further known that the formation of dense, pure inorganic coatings by the sol-gel route requires heating. The amount of heat necessary to produce dense, pure inorganic coatings makes it prohibitive to coat certain materials, such as plastic substrates. Additionally, pure inorganic coatings are brittle due to their high (three-dimensional) inorganic network connectivity.
The introduction of organic components to sol-gel compositions can lead to a reduction of overall inorganic network connectivity in coatings and allow coatings to be produced at lower temperatures. Also, the inclusion of organic components allows coatings to be more flexible viscoelastic (that is, less brittle) while maintaining good scratch resistance and abrasion resistance. Some of these inorganic/organic coatings have become known as ormosils (organically modified silicates), ormocer (organically modified ceramics), or nanomers (nanoparticle and organic component containing polymer type materials).
However, up to now these inorganic/organic coatings have suffered from a number of deficiencies. First, it has been extremely difficult, if not impossible in some cases, to produce coatings having an inorganic portion greater than about 40 percent by weight. Coatings having less inorganic content will have insufficient abrasion resistance or insufficient scratch resistance for many applications. Second, the inorganic/organic coating compositions of the prior art are either sensitive to water or immiscible in water, requiring organic solvents that may be expensive, difficult, or even hazardous to use.
Accordingly, there exists a need in the industry for hard coatings that can be easily and economically produced at low temperatures. The present invention addresses these concerns.
In one aspect, the present invention is composition that can be easily utilized to produce hard coatings. Compositions of the present invention comprise a)dispersion of inorganic particles in the composition, said particles having a size of from about 1 nanometer to about 100 nanometers; b) a non-hydrogen Lewis Acid, the weight percent of said Lewis Acid comprising from about 0.1 percent to about 30 percent of the total weight of the composition and c) an epoxy group-containing silicon compound dispersed substantially uniformly in the solution.
In one preferred embodiment, compositions of the present invention are aqueous dispersions. Although other liquid media may be used in dispersions of the present invention to produce coatings of the present invention, an important advantage of the present invention is that compositions of the present invention can be produced using only water as a dispersing medium. Preferably, the dispersions are substantially uniformly made in the composition.
The components in compositions of the present invention can be mixed in a variety of ways known in the art. A preferred method for mixing the components is to first prepare a dispersion containing the inorganic particles and add to it the epoxy group-containing silicon compound, after which is added the non-hydrogen Lewis Acid in the liquid medium. The compounds themselves or a solution containing the epoxy group-containing silicon compound or the non-hydrogen Lewis Acid can be used with a liquid medium being employed.
The method of producing coatings from the compositions of the present invention is not particularly critical. The coatable composition is first applied to a substrate to be coated and then the composition is cured to produce the cured coating composition on the substrate. Compositions of the present invention are cured in one of two ways depending on whether the solvent or liquid medium present needs to be removed or whether the solvent or liquid medium is reactable. The first method of curing is to first remove the solvent or liquid medium and then apply appropriate stimulus (for example, heat or UV light) to cross-link or polymerize the epoxy group-containing silicon compound. The second method of curing compositions of the present invention, used when a reactable liquid medium is present, is to react the liquid medium, instead of removing it, and cross-link or polymerize the epoxy group-containing silicon compound by applying appropriate stimulus.
A preferred method for curing compositions of the present invention, especially for coatable compositions containing aqueous dispersions, is to apply sufficient heat to the solution to remove the liquid medium and promote the cross-linking. Generally, compositions of the present invention can be cured at a temperature that is sufficiently low to allow compositions of the present invention to be cured on plastic substrates, such as substrates made of polycarbonate, polyethylene terephthalate (PET), polyethylenenaphthalenate (PEN), polyvinyl chloride (PVC), polymethylmethacrylate (PMMA), polyolefins, polysulfone, epoxy resins, polyurea and polyurethane.
In yet another aspect, the present invention is a novel composition that can be utilized as an abrasion-resistant coating. Compositions of the present invention comprise a) inorganic particles dispersed substantially uniformly throughout the composition, said particles having a size of from about 1 nanometer to about 100 nanometers, b) a non-hydrogen Lewis Acid or surface modifier dispersed substantially uniformly throughout the composition and c) an epoxy group-containing silicon compound.